Oh boy, this is a very tough question to answer. It’s like asking what is someone’s favorite movie of all time! Nevertheless, I will put my two cents in.
Thanks to thousands of research on core activation and stabilization, there’s a plethora of exercises available today that can effectively work and strengthen the entire core region. While all of the exercises recommended by everyone above are great, studies have shown that there are two exercises that effectively activate the entire core, including internal and external obliques, and they are the Stability Ball Roll-Out and Stability Ball Pike. Thanks to the masterminds at TRX, these two exercises can also be performed using a TRX which makes it a very advanced progression. When performed correctly, the Roll Out and Pike are perhaps two of the best all-time core activation exercises. Another great core stabilization exercises that is making waves across gyms is the Pallof Press (look it up on Google or YouTube), named after well renowned Physical Therapist John Pallof.
No longer are sit-ups and crunches the norms for abdominal/core activation, even though there are still a majority of exercise enthusiasts doing them in gyms. The best exercises are the ones that forces the core to stabilize in motion thus making the TRX, Stability Ball and Bosu Ball preferred tools in working the abs. However, I admire the creativity of a lot Personal Trainers in coming up with various exercises that don’t necessarily force the core to stabilize but yet effectively work the abdominal/core region.
I’ll say this in conclusion. You won’t know the exercise that works for you until you try it. So your favorite core exercise may very well be the one produces the best results!
Great question by the way!